Such micromachined inertial sensors produced on a silicon or quartz wafer are already known. The structure is planar in the plane of the silicon or quartz wafer in which it is etched.
Structures based on two vibrating masses mechanically coupled in the manner of a tuning fork have already been produced. The structure of a gyrometer thus produced typically comprises two coplanar moving masses that are excited in vibration and connected as a tuning fork, that is to say the two masses are connected to a central coupling structure that transfers the vibration energy from the first mass to the second mass, and vice versa.
The masses are excited into vibration in the plane of the wafer by an electrical excitation structure. This vibration in the plane of the wafer is exerted perpendicular to an axis called the “sensitive axis” of the gyrometer, perpendicular to the direction of this vibration. When the gyrometer rotates at a certain angular velocity about its sensitive axis, the composition of the forced vibration with the angular rotation vector generates, by the Coriolis effect, forces that set the moving masses into natural vibration perpendicular to the excitation vibration and to the axis of rotation; the amplitude of this natural vibration is proportional to the speed of rotation.
The natural vibration is detected by an electrical detection structure. The electrical signals that result therefrom are used to deduce from them a value of the angular velocity about the sensitive axis.
In certain cases the sensitive axis lies in the plane of the wafer and the detection structure detects a movement perpendicular to the plane of the moving masses. In other cases, the sensitive axis of the gyrometer is the axis Oz perpendicular to the plane of the wafer. The excitation movement of the moving masses is generated in a direction Ox of the plane, while a movement resulting from the Coriolis force is detected in a direction Oy, perpendicular to Ox, in the same plane.
The masses are capable of vibrating in two orthogonal vibration modes—the excitation mode, also called the primary mode, and the detection mode, also called the secondary mode.
The tuning-fork architecture has a drawback: the secondary mode is not in dynamic equilibrium. Consequently, this mode transmits a moment to the support of the tuning fork, which makes this mode sensitive to the conditions of attachment to the support and sensitive to the external perturbations transmitted by the support.
To remedy this problem, one solution consists in isolating the secondary mode using a double tuning-fork structure as shown in FIG. 1. The most well-known example is that of a Systron-Donner quartz double tuning-fork gyrometer.
The excitation movement parallel to Ox is provided by the upper fork as indicated in the figure and the sensitive axis is the Oy axis. The Coriolis moment created on the excitation fork generates a detection movement on the upper fork. An opposed detection movement is induced, by coupling, in the lower fork. This movement of the lower fork in phase opposition with that of the upper fork then allows the detection mode to be completely isolated. However, in such a gyrometer the detection movement lies out of the plane of the substrate. This has drawbacks such as, for example, the greater difficulty in controlling the orthogonality between the excitation movement and the detection movement, and a more complicated fabrication technology.